xfree86: Inline xf86{Read,Write}Mmio{8,16,32} on alpha

In commit 9db2af6f757e (xfree86: Remove xf86{Map,Unmap}VidMem) we somehow stopped exporting xf86{Read,Write}Mmio{8,16,32}. Since the function pointer indirection was intended to support dense vs sparse and sparse support is now gone, we can just make the functions static inline in compiler.h and avoid all of this.